In recent years, a storage system comprising a first and a second storage device different from each other in access speed has been developed. Here, let us suppose that the first storage device is fast in access speed but small in storage capacity, whereas the second storage device is slow in access speed but large in storage capacity. A storage system may be implemented by hierarchically combining the first storage device and the second storage device in such a manner that the first storage device may be positioned at a higher tier and the second storage device may be positioned at a lower tier. Such a technique is called a tiered storage technique, and a storage system to which the tiered storage technique is applied is called a tiered storage system.
In a tiered storage system, it is the characteristics, which each piece of data has, that determine and change which piece of data should be placed (stored) in which of the tiers (the storage devices). Specifically, a data piece which is high in access frequency should be placed in a higher tier (the first storage device) and another data piece which is low in access frequency should be placed in a lower tier (the second storage device). Such an arrangement (a tiered arrangement) will bring improvement in performance and suppression of cost to a storage system.
Generally, a tiered storage system automatically executes tiered classification. In order to automatically execute tiered classification, in a tiered storage system, a storage controller monitors an access status for every data set called an extent, for example. Generally, an extent has a size larger than a basic size (so called a block size) for data management (data access). An access status of each extent may be indicated by a statistic such as an access frequency.
The storage controller periodically evaluates every access frequency for every extent based on every access statistic, which every extent records, for example. Based on the periodical evaluation result, the storage controller transfers (relocates) pieces of data for every extent from a lower tier to a higher tier or from a higher tier to a lower tier. However, the data transfer between the tiers based on the periodical access frequency evaluation has a large overhead, and spends time on processing.